In the process of conveying fluids in pipelines, it is common to provide chemical treatments or other means to limit the rate of corrosion and monitor the pipeline to make sure that the corrosion inhibition program is effective. Monitoring corrosion rates have historically been accomplished by intrusive pipe or vessel wall penetrating instruments and/or metal coupons that are carefully weighed prior to installation and once again after retrieval. The instruments and coupons alter the internal flow of the fluids around or through the measuring devices which potentially generates environments less or more severe than if they were not present. Such conditions can produce corrosion data that is misleading. Further, while some coupons are made from the same materials as used in the construction of the pipe line or vessel, exact metallurgical replication of the material is suspect as well. In addition, local area galvanic and scale affects may not be suitably reproduced on the small surface area electronic probes. Typically, the coupons are withdrawn for measurements such as weight loss and the recovery of these instruments from an operating vessel or pipe requires special tools and careful procedures. If the corrosion rate is higher than expected, coupons have been lost inside the vessel or pipeline creating additional issues. Additionally, intrusive probes have a useful life and therefore must also be removed if another device is to takes its place and continue to provide the monitoring.
In addition, operating pipelines are often installed in places not easily accessible, perhaps by weather such as in the arctic, and regular access and monitoring get quite complicated. And once metal in the pipe has been corroded or pitted, the lost metal does not come back. For readily understandable cost issues related to buying extra thick and heavy pipe, transporting extra thick and heavy pipe to the pipeline location and welding and installing extra thick and heavy pipe, pipelines are generally not constructed with a lot of extra thickness. As such, early detection of an ineffective corrosion inhibitor program is important for long term use of the pipeline.
Such issues were broadly revealed to the public in the news reports of the 2006 leaks in the pipelines in Prudhoe Bay, Ak. where unexpectedly high corrosion was found and the pipeline was taken out of service for months while many miles of pipe had to be replaced. The time period during which the inside of pipeline was not inspected by a pipeline inspection gauge (“pig”) was too long and other testing technologies were not used on the pipe. The operator trusted the corrosion control protocols for the pipeline without double checking the corrosion rate with adequate measurements. Running a pig is not a simple or low cost exercise and other techniques for monitoring corrosion techniques are highly desired.
One non-invasive technique for measuring corrosion is an ultrasonic measuring device which directs ultrasonic energy into the pipeline. As the sensor receives reflections from the back or inside wall of the pipe and the elapsed time from sending to receiving, or two or more sequential backwall echoes, provides a measure of the wall thickness at that location. Currently, field use ultrasonic sensors are primarily handheld devices allowing for many measurements at diverse locations, but only taken on a periodic basis. Such sensors provide an accuracy of up to about plus or minus 1 mil (0.001 inches) and is typically quoted at an accuracy of plus or minus 10 mils by corrosion engineers. While this may sound accurate, a pipe having a thickness of ¼ inch is only 250 mils thick. A pipe with ⅛ inch wall is only 125 mils thick. The Prudhoe Bay pipe was ⅜ inch thick and was found to be approximately 70 to 80 percent corroded. Pipes for pipelines are not made with high precision, and as such, the thickness of pipes vary by several mils immediately after manufacture in all directions (along length and around the periphery. As such, measureable corrosion is generally not detected by two successive measurements of a handheld device due to measurements at slightly different locations, by different personnel and possibly at different temperatures until at least 10 mils of pipe thickness is lost and as much as 20 or more mils of wall thickness are actually lost. Clearly, it would be more desirable to identify unacceptable loss or an unacceptable loss rate at a much earlier time frame before much damage is sustained.